# Checker

Like we mentioned before *checker* is the thing that makes TypeScript uniquely more powerful than *just another JavaScript transpiler*. The checker is located in `checker.ts` and at this moment it is 23k+ lines of TypeScript (largest part of the compiler).

## Usage by Program

The `checker` is initialized by `program`. The following is a sampling of the call stack (we showed the same one when looking at `binder`):

```
program.getTypeChecker ->
    ts.createTypeChecker (in checker)->
        initializeTypeChecker (in checker) ->
            for each SourceFile `ts.bindSourceFile` (in binder)
            // followed by
            for each SourceFile `ts.mergeSymbolTable` (in checker)
```

## Association with Emitter

True type checking happens once a call is made to `getDiagnostics`. This function is called e.g. once a request is made to `Program.emit`, in which case the checker returns an `EmitResolver` (program calls the checkers `getEmitResolver` function) which is just a set of functions local to `createTypeChecker`. We will mention this again when we look at the emitter.

Here is the call stack right down to `checkSourceFile` (a function local to `createTypeChecker`).

```
program.emit ->
    emitWorker (program local) ->
        createTypeChecker.getEmitResolver ->
            // First call the following functions local to createTypeChecker
            call getDiagnostics ->
                getDiagnosticsWorker ->
                    checkSourceFile

            // then
            return resolver
            (already initialized in createTypeChecker using a call to local createResolver())
```


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